AU2004259849A1

AU2004259849A1 - Resistant alloy for heat exchangers

Info

Publication number: AU2004259849A1
Application number: AU2004259849A
Authority: AU
Inventors: Wolf-Dieter Finkelnburg; Lothar Lochte; Raimund Sicking; Pascal Wagner
Original assignee: Hydro Aluminium Deutschland GmbH
Current assignee: Hydro Aluminium Deutschland GmbH
Priority date: 2003-07-25
Filing date: 2004-07-26
Publication date: 2005-02-03
Also published as: BRPI0412907A; KR20060030910A; CA2533428A1; EP1649070A1; EP1505163A3; JP2007500784A; WO2005010223A1; EA200600211A1; EP1505163A2

Description

VERIFICATION OF TRANSLATION 1, Go &. . . .I . ............ of . ........... verify that the following is a true translation of International Patent Application No. PCT/EP2004/008359 to the best of my knowledge and belief. DATED this ..A.6.. day of .... . - rCV............ 2006 GS 030029WO HIGH-STRENGTH ALLOY FOR HEAT EXCHANGERS The invention relates to a cold age-hardenable aluminium alloy for heat exchangers, a method for producing a cold age-hardenable aluminium strip and an aluminium strip or sheet. Heat exchangers consisting of aluminium or aluminium alloys are increasingly being used in the automobile field. In this case, the use of aluminium instead of the previously commonly used nonferrous metal heat exchangers has almost halved the weight of heat exchangers with comparable size and performance. Heat exchangers of aluminium or an aluminium alloy are nowadays used in the motor vehicle mainly for cooling the cooling water, oil and in air-conditioning systems. Heat exchangers for motor vehicles are usually made of aluminium strips or sheets, the individual pre-fabricated components of the heat exchanger such as fins, tubes and distributors for example, being joined together by brazing. The loads acting in practical use on components thus manufactured and built into motor vehicles as a result of intermittent vibrations, more sustained vibrations, corrosion attack and similar are considerable. This particularly applies to the fins via which the heat is removed. Despite the considerable loads and increasing operating pressures of the heat exchangers in motor vehicles, there continues to be a trend towards saving weight in motor vehicles and therefore towards a further reduction in the wall thickness of the heat exchanger. However, this results in further increasing strength requirements for the aluminium alloy of the heat exchangers, especially after brazing. On the one hand, vacuum brazing without flux and - 2 inert-gas brazing using non-corrosive fluxes are available for brazing heat exchangers. The cold age hardenable aluminium alloys used so far for the vacuum brazing of heat exchangers, for example, the aluminium alloy AA6063 (AIMgO, 7Si), AA6061 (AlMglSiCu) or AA6951 (AIMgO, 6SiCu), have relatively high magnesium contents in order, on the one hand, to prevent any oxidation of the molten aluminium solder on the components to be brazed as a result of "gettering" during the brazing process in vacuum and thereby ensure a perfect brazed joint without flux and on the other hand, to achieve high strength values of the brazed heat exchangers during natural ageing after brazing. A disadvantage with this method however is that it is cost-intensive to maintain the gas protection and the purity requirement for the components to be soldered. The alternative inert-gas brazing (also called CAB - controlled atmosphere brazing) certainly requires less expenditure under these aspects and additionally makes it possible to achieve up to 20% shorter brazing cycles but it is not possible to use the aluminium alloy having high magnesium contents known from vacuum brazing since the magnesium reacts with the non corrosive fluxes during the brazing. This can only be prevented by using more expensive caesium-containing fluxes. It is furthermore possible to use high-copper content aluminium alloys (Cu content > 0.5%) which however tend to form heat cracks during casting and thus impose increased requirements on the casting of the rolling bars which should be considered to be critical from economic aspects. In addition, at elevated Cu contents there is a risk of sensitivation for pitting or grain-boundary corrosion if copper is present in suitably precipitated form in the structure. Finally, in inert-gas - 3 brazing an aluminium alloy with an intermediate cladding can be used as a diffusion barrier layer so that a cold age-hardenable aluminium alloy with relatively high magnesium contents is used as core material. However, intermediate cladding with a diffusion barrier layer is associated with additional costs so that economical production of heat exchangers likewise cannot be achieved. The fabrication of heat exchangers by brazing of components consisting of the aforementioned aluminium alloys is known, for example, from US Patent Specification US 4,214,925. Starting from the previously indicated prior art, it is thus the object of the present invention to provide a cold age-hardenable aluminium alloy for heat exchangers, a method for producing an aluminium strip for heat exchangers and a corresponding aluminium strip or sheet which has high strength values after natural ageing following brazing. According to a first teaching of the present invention, the object derived and indicated above is solved for an aluminium alloy by said aluminium alloy having the following alloying components in wt.%: Si < 0.7% 0.1% Mg 1% Fe < 0.3% 0.08% Cu 0.2% Ti < 0.2% Mn < 0.1% Cr < 0.1% - 4 Zn 0.1%, unavoidable accompanying elements individually to a maximum of 0.1% and in total to a maximum of 0.15% and aluminium as the remainder. It has surprisingly been shown that heat exchangers consisting of an aluminium alloy containing the alloying fractions specified above, after natural ageing at room temperature following brazing, have the necessary strength for use in motor vehicles, especially the yield point RP 0 .2, without further heat treatments being necessary. The reason for this is the combination of the Si and Mg contents according to the invention which form finely distributed precipitates of the type Mg 2 Si in the aluminium alloy according to the invention and result in an increase in strength as a result of natural ageing at room temperature. This increase in strength by natural ageing is further improved by adding copper in the claimed range of 0.08 wt.% to 0.2 wt.%. Limiting the Fe content to a maximum of 0.3 wt.% ensures that Si is present in the aluminium alloy in a dissolved state. Furthermore, the low Cu contents of 0.2 wt.% maximum on the one hand ensure that the increase in strength during natural ageing can be increased and on the other hand, this limitation of the Cu content reduces the sensitivity of the strength of the aluminium alloy to the cooling rate after brazing. Likewise, the Mn content must be limited to a maximum of 0.1 wt.% to limit the dependence of the strength of the aluminium alloy on the cooling rate after brazing. In contrast, Cr contents of 0.1 wt.% maximum increase the strength and the corrosion resistance of the aluminium alloy according to the - 5 invention. In addition, a Ti content of 0.2 wt.% maximum has a positive effect on the resistance to corrosion of the aluminium alloy according to the invention since the Ti alloying element contributes to the grain refining of the structure of the aluminium alloy and thus makes the corrosion attack uniform. In order to avoid the negative effect of zinc on the corrosion of the aluminium alloy according to the invention, the Zn content must be restricted to a maximum of 0.1 wt.%. According to a first advantageous embodiment, the strength of the aluminium alloy according to the invention can be further increased by natural ageing after brazing by the aluminium alloy containing Si, Mg and Cu as the principal alloying elements. In order to avoid softening of the components of a heat exchanger to be brazed during brazing, it is advantageous for carrying out a perfect brazing process if the solidus temperature of the aluminium alloy does not go below 610 0 C since brazing is usually carried out at temperatures up to 600 0 C. According to the invention this is achieved by the total of the alloying fractions of Si, Mg and Cu not exceeding 1.2 wt.%. In this case, alloying elements generally bring about a reduction in the solidus temperature where Si causes a reduction in the solidus temperature of the aluminium alloy a factor of 1.2 greater than Mg and Mg in turn causes a reduction in the solidus temperature a factor of 3.5 more effective than Cu. This does not apply to the alloying element Ti so that an increase in the solidus temperature of the aluminium - 6 alloy according to the invention can be achieved by the aluminium alloy having Ti as an alloying component. If the upper limit of the claimed alloy is exhausted for magnesium, the brazing of heat exchangers fabricated from this alloy is preferably effected by vacuum brazing. Inert-gas brazing using caesium-containing fluxes is also possible here to a limited extent. Inert-gas brazing using caesium-containing fluxes is especially simplified by the alloying fraction of magnesium not exceeding 0.8 wt.%. In addition, with a low Mg content up to a maximum of 0.3 wt.%, the aluminium alloy according to the invention is readily suitable for inert-gas brazing using non corrosive fluxes since a reaction with the fluxes only takes place to a limited extent and the use of more expensive caesium-containing fluxes can be dispensed with. A particularly advantageous embodiment of the aluminium alloy according to the invention is obtained whereby after processing and brazing and after natural ageing for approximately 30 days at room temperature the aluminium alloy has particularly high strength values. This material property ensures a particularly inexpensive fabrication process since the natural ageing as part of the transport process already ensures a very good strength without further measures. According to a second teaching of the present invention, the object derived and indicated above is solved according to the method by - 7 - a rolling bar being cast from an aluminium alloy according to the invention in a conventional bar casting method, - the rolling bar being homogenised at 500 to 600 0 C for more than 6 h, especially for more than 12 h, and being hot-rolled at at least 4000C, preferably 450'C, to form a strip, wherein the final temperature during the hot rolling is at least 3000C, - the hot-rolled strip being cold-rolled to final thickness and being then subjected to soft annealing at at least 3000C, preferably 3500C. As a result of the homogenisation of the rolling bar cast by the conventional bar casting method at temperatures of 500 to 6000C for more than 6 hours, especially for more than 12 hours, it is achieved that even sluggishly diffusing elements such as manganese and chromium are precipitated in a finely dispersed fashion during cooling of the melt. As a result of the hot rolling at at least 4000C an optimised structure of the hot strip is produced with regard to the deformability and corrosion resistance where the final rolling temperature during hot rolling must be at least 3000C in order to achieve sufficient deformability of the rolling bar on the one hand and optimised structure formation during the hot rolling on the other hand. In this case, the hot strip final thickness can be less than 9 mm, for example. In order to facilitate the forming of the strip produced by the method according to the invention into pre-fabricated - 8 components for the heat exchangers, for example, fins, tubes or distributors, the strip which has been cold rolled to a maximum final thickness of 2 mm by cold rolling is subjected to subsequent soft annealing at at least 300 0 C, preferably 350 0 C. As a result of the combination of the alloy composition of the aluminium alloy in conjunction with the process features described previously, heat exchangers can be fabricated on the basis of conventional alloying elements (Mg, Si, Cu) which, after inert-gas brazing and natural ageing for about 30 days at room temperature, have yield points of RP 0

.

2 65 MPa and are thus particularly well suited for the enormous loads in motor vehicles. In addition, inert-gas brazing without using caesium containing fluxes can be used to fabricate the heat exchangers so that economical fabrication is possible. If the hot rolling and/or cold rolling takes place in a reversing or unidirectional mode on single- or multiple stand rollers, the method according to the invention can be carried out using conventional means and devices with regard to the reducing rolling. A particularly high process safety during brazing of the heat exchanger can be achieved by cladding the rolling bar with an aluminium solder after homogenising. The aluminium strip fabricated from this rolling bar has a uniform layer of aluminium solder which during brazing, results in particularly homogeneous and uniform brazed joints, for example, between the fins, tubes and distributors of the heat exchanger. If only one side of the aluminium strip according to the invention is clad - 9 with an aluminium solder, the other side can be clad or coated with an alloy serving as corrosion protection, for example. Advantageously used as aluminium solder is an aluminium alloy having a silicon content of 6-13 wt.%, especially an AISi7 or AlSilO alloy, which during inert-gas brazing have a particularly good wetting capacity with aluminium solder with regard to the oxide layers remaining in non oxidising atmospheres on the components of the heat exchanger to be brazed. Finally, the object derived and indicated above is solved according to a third teaching of the present invention by an aluminium strip or sheet for fabricating heat exchangers which is produced by the method according to the invention. As has already been stated, an aluminium strip or sheet produced by the method according to the invention has improved strength values, especially yield point, after natural ageing following brazing so that the wall thicknesses of the heat exchanger can be further reduced. In addition, inert-gas brazing using non corrosive fluxes can be used to fabricate the heat exchangers without using caesium-containing fluxes. The aluminium strip or sheet advantageously has a maximum thickness of 2 mm, especially 1 mm. As a result of the higher strength compared with conventional materials, when using the aluminium strip according to the invention, the strip thickness can be further reduced and thus material can be saved during the fabrication of heat exchangers and a further reduction in the weight of the heat exchangers can be achieved. In this case, the - 10 operating safety of the heat exchanger is not impaired, even at higher operating pressures, because of the higher strength of the aluminium alloy. There are now a plurality of possibilities for configuring and further developing the cold age hardenable aluminium alloy for heat exchangers according to the first teaching of the invention, the method for producing a cold age-hardenable aluminium strip for heat exchangers according to the second teaching of the invention and the aluminium strip or sheet according to the invention for fabricating heat exchangers according to the third teaching of the invention. For this purpose, for example, reference is made on the one hand to the claims subordinated to claims 1, 5 and 9, on the other hand to the description of an exemplary embodiment of a method for fabricating a cold age-hardenable aluminium strip for heat exchangers according to the second teaching of the invention in conjunction with the drawing. In the drawing the single figure is a schematic diagram showing the production path for implementing an exemplary embodiment of a method for fabricating a cold age hardenable aluminium strip for heat exchangers according to the second teaching of the invention. The production path shown in the single figure comprises the bar casting 1 from an aluminium alloy in a first step. In this case, the aluminium alloy of the exemplary embodiment has the following alloying components in wt.% - 11 0.60% Si < 0.70%, 0.12% Fe 0.30%, 0.08% Cu < 0.20%, 0.04% - Mn K 0.08%, 0.12% Mg K 0.30%, Cr < 0.05%, Zn < 0.05%, 0.08% Ti < 0.20%, B < 50 ppm, unavoidable accompanying elements to a maximum of 0.03% and to a maximum of 0.1% in total and aluminium as the remainder. The low boron content of 50 ppm maximum improves the recyclability of the aluminium alloy. The rolling bars cast using the DC method from the aluminium alloy just described are then homogenised in a homogenisation stage 2. Particularly good results with regard to the homogenisation of the rolling bar were achieved at a temperature of 575 0 C for more than 6 h, especially 12 h. Following homogenisation the rolling bars are then hot rolled on a tandem stand 3a to a thickness of 7 mm, for example, wherein in particular the final temperature during hot rolling must be higher than 300 0 C, preferably 330 0 C, in order to ensure optimised structure formation during the hot rolling. Alternatively, however, the hot rolling can be carried out on a reversing stand 3 and wound onto a reel which is not shown and hot rolling in the tandem stand 3a can be dispensed with. The subsequent cold rolling to a final thickness of about 1 mm takes place on single- or multiple-stand rollers 4. Like the hot rolling the cold rolling can alternatively also be - 12 carried out in reversing mode on a reversing stand. As a result of final soft annealing at about 350 0 C in a batch furnace 5, the aluminium strip is converted to a state of lowest possible strength and high elongation to facilitate subsequent forming work during fabrication of the heat exchanger components. Alternatively to the exemplary embodiment of the method according to the invention for producing a strip for heat exchangers which has just been described, after homogenisation in the homogenisation stage 2 the rolling bar can be clad with an aluminium solder, for example of an AlSi7 or AlSil0 alloy, to avoid subsequent application of an aluminium solder before the brazing of the heat exchangers fabricated from the strip according to the invention. For this purpose the rolling bar must be heated to an initial rolling temperature of at least 400 0 C, preferably 450'C, before the hot rolling. When brazing heat exchangers fabricated from aluminium strip or sheet according to the invention, especially when using inert-gas brazing particularly high strength values of the heat exchanger, in particular values for the yield point of RP 0

.

2 2 65 MPa can be achieved without using caesium-containing fluxes at temperatures up to 600 0 C and typical cooling rates of 30 0 C/min from 600 0 C to 200'C as well as natural ageing of about 30 days at room temperature after brazing. The cooling from 200 0 C to room temperature need not to take place in an exactly defined manner.

Claims

1. A cold age-hardenable aluminium alloy for heat exchangers, characterised in that the aluminium alloy comprises the following alloying components in wt.%: Si < 0.7% 0.1% Mg < 1% Fe 0.3% 0.08% Cu 0.2% Ti 0.2% Mn 0.1% Cr 0.1% Zn 0.1%, unavoidable accompanying elements individually to a maximum of 0.1% and in total to a maximum of 0.15% and aluminium as the remainder.

2. The cold age-hardenable aluminium alloy according to claim 1, characterised in that the aluminium alloy contains Si, Mg and Cu as principal alloying elements.

3. The cold age-hardenable aluminium alloy according to claim 1 or claim 2, characterised in that the total of the alloying fractions of Si, Mg and Cu does not exceed 1.2 wt.%.

4. The aluminium alloy according to any one of claims 1 to 3, characterised in that the aluminium alloy comprises Ti as an alloying component. - 2

5. The aluminium alloy according to any one of claims 1 to 4, characterised in that the alloying fraction of Mg does not exceed 0.8 wt.%.

6. The aluminium alloy according to any one of claims 1 to 4, characterised in that the alloying fraction of Mg does not exceed 0.3 wt.%.

7. The aluminium alloy according to any one of claims 1 to 6, characterised in that after processing and brazing and after natural ageing for approximately 30 days at room temperature the aluminium alloy has particularly high strength values.

8. A method for producing a cold age-hardenable aluminium strip for heat exchangers from an aluminium alloy according to any one of claims 1 to 7, characterised in that - a rolling bar is cast in a conventional bar casting method, - the rolling bar is homogenised at 500 to 600 0 C for more than 6 h, especially for more than 12 h, - the rolling bar is hot-rolled at at least 400 0 C, preferably 450 0 C, to form a strip, wherein the final temperature during the hot rolling is at least 300 0 C, - 3 - the hot-rolled strip is cold-rolled to final thickness and is then subjected to soft annealing at at least 300 0 C, preferably 350 0 C.

9. The method according to claim 8, characterised in that the hot rolling and/or cold rolling takes place in a reversing or unidirectional mode on single- or multiple-stand rollers.

10. The method according to claim 8 and claim 9, characterised in that after homogenising the rolling bar is clad with an aluminium solder.

11. The method according to claim 10, characterised in that an aluminium alloy having a silicon content of 6-13 wt.%, especially an AlSi7 or AlSil0 alloy, is used as aluminium solder.

12. An aluminium strip or sheet for manufacturing heat exchangers consisting of an aluminium alloy according to any one of claims 1 to 7, especially manufactured using a method according to any one of claims 8 to 11.

13. The aluminium strip or sheet according to claim 12, characterised in that the aluminium strip or sheet has a maximum thickness of 2 mm, preferably 1 mm.